There has been a problem that an impulsive noise mixes due to a device fault, etc. at the time of A/D or D/A conversion in a voice communication network. In the Next Generation Network (NGN), a high communication quality is demanded, and it is necessary to detect and remove an impulsive noise.
The following two methods are known as techniques for detecting an impulsive noise.    (1) Detecting an impulsive noise by using the phenomenon that the amplitude or the amplitude fluctuation of a signal becomes large in a portion where an impulsive noise exists.    (2) Detecting an impulsive noise by using the phenomenon that the frequency characteristic of an impulsive noise differs from a voice signal.
According to Patent Document 1, an impulsive noise is detected by using the above described conventional detecting method (1). Namely, the absolute value of a difference between the amplitudes of adjacent PCM codes is measured, and a click noise is determined if the measured value is larger than a threshold value (paragraph 0007). The threshold value on the order of 1/16 to ⅛ (2000 to 4000 for 16-bit PCM) of the maximum allowable amplitude is used.
FIG. 1A illustrates an impulsive noise, whereas FIG. 1B illustrates a tone signal. If the threshold value of the difference between the maximum value of the amplitude of the impulsive noise and the amplitude of the signal is set to 2000 to 4000, the absolute value of the difference between the amplitudes of the impulsive noise and the signal becomes larger than the threshold value (2000 to 4000). Therefore, the impulsive noise can be detected.
However, for the 3-kHz tone signal, which is illustrated in FIG. 1B and the average power of which is on the order of −25 dBov, the amplitude difference becomes approximately 3000 at the maximum, and a normal signal can be possibly determined as an impulsive noise in an erroneous manner depending on a threshold value. Square dots illustrated in FIG. 1B represent sampling points.
FIGS. 2A, 2B and 2C respectively illustrate a 1-kHz tone signal, an impulsive noise, and the tone signal on which the impulsive noise is superimposed.
Assume that the maximum value of the amplitude of the 1-kHz tone signal illustrated in FIG. 2A is 1000, the peak value of the impulsive noise is 1000, and the impulsive noise occurs when the value of the tone signal is negative. In this case, the amplitude difference in the portion of the impulsive noise becomes equal to or smaller than the threshold value. Therefore, the impulsive noise cannot be detected with the conventional detecting method (1).
FIG. 3 is a block diagram of a conventional noise detecting circuit 11. FIGS. 4A to 4D illustrate the signal waveforms of the noise detecting circuit 11. The noise detecting circuit 11 detects an impulsive noise by using the above described conventional method (2).
A low-frequency component of an input signal is cut off by a highpass filter (HPF) 12. As a result, the high-frequency signal illustrated in FIG. 4B is extracted from the input signal which is illustrated in FIG. 4A and on which a high-frequency noise is superimposed.
A rectifying circuit 13 rectifies a high-frequency signal. With the rectifying circuit 13, only a positive component of the high-frequency signal, which is illustrated in FIG. 4C, is output.
A lowpass filter (LPF) 14 outputs an envelope signal of the output signal of the rectifying circuit 13. With the lowpass filter 14, the envelop signal illustrated in FIG. 4D can be obtained.
A signal comparing unit 15 makes a comparison between the high-frequency signal output from the highpass filter 12 and the envelope signal output from the lowpass filter 14, and determines an impulsive noise if the high-frequency signal is larger than the envelope signal by a predetermined value or more.
Patent Document 2 recites that a music signal similar to a pulsed noise is prevented from being erroneously determined as a noise by making a periodical determination with an autocorrelation function. Patent Document 2 also recites that the pulsed noise is replaced with a prediction signal generated with a linear prediction method.
However, the invention disclosed by Patent Document 2 targets an impulsive noise superimposed on a music signal when a record is reproduced, and assumes a high sampling frequency. For a signal of low sampling frequency (for example, 8-kHz sampling frequency) such as a voice communication, a noticeable signal power difference does not occur in the frequencies of an impulsive noise and a voice signal. Therefore, the impulsive noise cannot be detected with signal powers.
FIG. 5 illustrates the frequency characteristics of the impulsive noise and the voice signal. As illustrated in FIG. 5, differences between the signal powers of the impulsive noise and the voice signal are small in a 0 to 4-kHz band. Therefore, the impulsive noise cannot be detected with the signal powers.
[Patent Document 1] Japanese Patent No. 3183490
[Patent Document 2] Japanese Laid-open Patent Publication No. 2006-178486